This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Patent Application 2000-243561, filed on Aug. 11, 2000, and Japanese Patent Application 2001-204225, filed on Jul. 5, 2001, the entire content of both of which is incorporated herein by reference.
This invention generally relates to a hydraulic brake device. More particularly, the present invention pertains to hydraulic brake device in a vehicle that is provided with an assisting device for assisting master cylinder actuation in response to brake pedal operation.
A known negative pressure booster or servo motor for use in a power braking system is disclosed in U.S. Pat. No. 3,910,048 issued in 1975. The servo motor includes a pressure ratio changer in which a first piston and a second piston supply a master cylinder with an operative force. The first piston is connected with a wall within the servo motor which is moved by a pressure differential across the wall. The second piston is concentrically positioned within the first piston to transmit an input force which operates a control valve to produce the pressure differential and also operates a hydraulic lock valve located within the master cylinder. As the first piston and the second piston move together in response to movement of the wall, hydraulic fluid flows into a locking chamber past the lock valve. When the output force capable of being generated by the pressure differential reaches a maximum value, the simultaneous movement of the first and the second pistons ceases. Further manual force input from the operator moves the second piston within the first piston to close the lock valve and hold the hydraulic fluid within the locking chamber to prevent the first piston from moving.
More specifically, as the first and the second pistons move forward, hydraulic pressure develops in the pressure chambers of the master cylinder. The output from the first and the second pistons follows a line (line 188 shown in FIG. 3 of U.S. Pat. No. 3,910,048) until the entire second chamber in the servo motor contains air at atmospheric pressure. At a particular point (point 190 in FIG. 3 of the aforementioned patent), the input force from the pedal is transmitted through a plunger and a sleeve to the second piston. This input force causes the second piston to independently move and permits a valve spring to close a hydraulic passage. As the second piston moves further, the output follows a different line (line 192 shown in FIG. 3 of the patent).
In the event a negative pressure is unavailable at the intake manifold, an input force applied to the brake pedal initially moves the second piston within the first piston to permit immediate closure of the hydraulic passage by the lock valve. The output from the second piston follows another line (line 196 shown in FIG. 3 of the patent) which exhibits a higher output than the characteristic line where both pistons are moved during a no power condition.
The servo motor for use in a power braking system disclosed in the aforementioned patent is configured to provide an output following the line 192 in FIG. 3 of the patent by enclosing the increased hydraulic pressure in the locking chamber in response to the movement of the second piston within the first piston after reaching the point 190. Even when the servo motor fails (e.g., a negative pressure is unavailable), the output can follow the line 196. To provide an output following the line, the servo motor serving as an assisting device needs to be configured for assuring a sufficient amount of movement of the second piston relative to the first piston. Accordingly, the structure of the known servo motor requires substantial change and an increase in the cost associated with the servo motor is unavoidable. Similarly, when the braking force is further increased after the output force of the servo motor reaches the maximum value, the structure of the known servo motor still needs to be changed.
A need thus exists for an improved hydraulic brake device for a vehicle having an assisting device that drives a master cylinder in response to a brake pedal operation.
It would be desirable to provide a hydraulic brake device for a vehicle that is able to assure a proper input-output performance after the output force of the servo motor reaches the maximum value in a manner that involves relatively minimal structural changes in the assisting device.
According to one aspect of the present invention, a hydraulic brake device for a vehicle includes a reservoir containing brake fluid, a master cylinder having a master piston for increasing a pressure of the brake fluid from the reservoir to supply pressure to a wheel cylinder in response to brake pedal depression, an assisting device for assisting the actuation of the master piston in response to the depression of the brake pedal, and an auxiliary piston provided rearward of the master piston and including first and second pistons. The assisting device includes a first output member for imparting an assisting output force of the assisting device passing through a reaction device and transmitted with an input force from the brake pedal passing through the reaction device. The assisting device also includes a second output member for imparting the assisting output force of the assisting device without passing through the reaction device. The first piston is operatively connected to the second output member of the assisting device and includes a first diameter portion having a diameter larger than that of the master piston. The second piston includes a first diameter portion having a diameter the same as that of the master piston and a second diameter portion smaller than the first diameter portion of the second piston. The second diameter portion of the second piston is provided in front of the first diameter portion of the second piston and has a diameter smaller than that of the master piston. The first diameter portion of the second piston is concentrically positioned in the first piston and the second diameter portion of the second piston is concentrically positioned in the master piston. The second piston is operatively connected to the first output member of the assisting device. A first hydraulic pressure chamber is provided between the first piston and the master piston and between the first diameter portion of the second piston and the master piston. A second hydraulic pressure chamber is provided between the second diameter portion of the second piston and the master piston. A first valve device interrupts hydraulic communication between the first hydraulic pressure chamber and the reservoir to hydraulically close the first hydraulic pressure chamber when actuation of the master cylinder by the assisting device is started. The first valve device also alternatively opens and closes hydraulic communication between the first hydraulic pressure chamber and the reservoir to decrease the hydraulic pressure in the first hydraulic pressure chamber in response to increasing the input force from the brake pedal when the input force from the brake pedal is further increased after the assisting output force of the assisting device reaches the maximum value. A second valve device hydraulically connects the second hydraulic pressure chamber with the first hydraulic pressure chamber when the hydraulic pressure in the first hydraulic pressure chamber is below a predetermined pressure less than a maximum hydraulic pressure generated by the assisting output force imparted from the assisting device to the first piston of the auxiliary piston. The second valve device also hydraulically closes the second hydraulic pressure chamber when the hydraulic pressure in the first hydraulic pressure chamber is above the predetermined pressure.
When the master piston is assisted through the auxiliary piston by the assisting device, the first hydraulic pressure chamber is hydraulically sealed by the first valve device. Then the auxiliary piston and the master piston are hydraulically connected. The brake pressure corresponds to the pressure in first and second hydraulic pressure chambers. After the pressure in the first hydraulic pressure chamber reaches the predetermined pressure, the second hydraulic pressure chamber can be hydraulically sealed. When the assisting output force of the assisting device reaches the maximum value, the first hydraulic pressure chamber is alternatively connected to and disconnected from the reservoir to decrease the pressure in the first hydraulic pressure chamber in response to an increase of the input force from the brake pedal, thereby causing a decrease of the force transmitted from the assisting device to the first piston of the auxiliary piston, and also causing an increase of the force transmitted from the assisting device to the second piston of the auxiliary piston so that the assisting operation of the assisting device is continued until the pressure in the first hydraulic pressure chamber is decreased at the atmospheric pressure. A brake pressure increase gradient in the assisting operation of the assisting device after the assisting output force of the assisting device reaches the maximum value corresponds to or is the same as the brake pressure increase gradient in the assisting operation of the assisting device before the assisting output force of the assisting deice reaches the maximum value, because the diameter of the second diameter portion of the second piston of the auxiliary piston corresponds to the diameter of the master piston.
The hydraulic brake device further includes a first hydraulic passage formed in the auxiliary piston for connecting the first hydraulic pressure chamber with the reservoir. The first valve device includes a first valve seat disposed in the first hydraulic passage, a first valve body adapted to be seated on or separated from the first valve seat for closing or opening the first hydraulic passage, a first biasing means always biasing the first valve body toward the first valve seat, and a brake input transmitting member for driving the first valve body in response to an increase of the input force from the brake pedal after the output force of the assisting device reaches the maximum value.
The hydraulic brake device also includes a second hydraulic passage formed in the auxiliary piston for connecting the second hydraulic pressure chamber with the first hydraulic pressure chamber. The second valve device includes a second valve body adapted to be alternatively seated on and separated from the second valve seat for alternatively closing and opening the second hydraulic passage, a second biasing means always biasing the second valve body toward the second valve seat, a pressure responsive member for moving the second valve body in a direction separated from the second valve seat in response to the pressure in the first hydraulic pressure chamber, and a third biasing means for always biasing the pressure responsive member toward the second valve body with a biasing force larger than that of the second biasing means.
The assisting device can be in the form of a negative pressure booster serving as a negative pressure assisting device or a hydraulic pressure booster serving as a hydraulic pressure assisting device.
According to another aspect of the invention, a hydraulic brake device for a vehicle includes a reservoir containing brake fluid, a master cylinder having a master piston for increasing a pressure of the brake fluid from the reservoir to supply brake pressure to a wheel cylinder in response to depression of a brake pedal, an assisting device for assisting actuation of the master piston in response to depression of the brake pedal, and an auxiliary piston provided rearward of the master piston. The assisting device includes a first output member for imparting an assisting output force of the assisting device passing through a reaction device and transmitted with an input force from the brake pedal passing through the reaction device, and a second output member for imparting the assisting output force of the assisting device without passing through the reaction device. The auxiliary piston includes first and second pistons, with the first piston including a first diameter portion having a diameter larger than that of the master piston and operatively connected to the second output member of the assisting device, and the second piston including a first diameter portion having a diameter the same as that of the master piston and a second diameter portion provided in front of the first diameter portion of the second piston. The second diameter portion of the second piston has a diameter smaller than that of the master piston, the first diameter portion of the second piston is concentrically positioned within the first piston, and the second diameter portion of the second piston is concentrically positioned within the master piston. The second piston is also operatively connected to the first output member of the assisting device. A first hydraulic pressure chamber is positioned between the first piston and the master piston and between the first diameter portion of the second piston and the master piston, and a second hydraulic pressure chamber is located between the second diameter portion of the second piston and the master piston. A first valve device controls hydraulic communication between the first hydraulic pressure chamber and the reservoir in response to operation of the brake pedal when actuation of the master piston by the assisting device through the auxiliary piston is started, and a second valve device controls hydraulic communication between the first hydraulic pressure chamber and the second hydraulic pressure chamber in response to hydraulic pressure in the first hydraulic pressure.
In accordance with another aspect of the invention, a hydraulic brake device for a vehicle includes a reservoir containing brake fluid, a master piston for increasing a pressure of the brake fluid from the reservoir in response to depression of a brake pedal, an assisting device for assisting actuation of the master piston in response to depression of the brake pedal, and an auxiliary piston provided rearward of the master piston. The assisting device includes a first output member for applying an assisting output force of the assisting device through a reaction member, and a second output member for imparting the assisting output force of the assisting device without passing through the reaction member. The auxiliary piston includes a first piston operatively connected to the second output member of the assisting device and a second piston operatively connected to the first output member of the assisting device. A first hydraulic pressure chamber is located between the first piston and the master piston, and a second hydraulic pressure chamber is located between the second piston and the master piston. A first valve device controls hydraulic communication between the first hydraulic pressure chamber and the reservoir in response to operation of the brake pedal when actuation of the master piston by the assisting device through the auxiliary piston is started, and a second valve device controls hydraulic communication between the first hydraulic pressure chamber and the second hydraulic pressure chamber in response to hydraulic pressure in the first hydraulic pressure. The hydraulic brake device also includes means for producing a brake pressure increase gradient in assisting operation of the assisting device after the assisting output force of the assisting device reaches the maximum value that is the same as the brake pressure increase gradient in assisting operation of the assisting device before the assisting output force of the assisting device reaches the maximum value.